1. Field of the Invention
The present invention relates to a mechanical quantity sensor configured to detect a mechanical quantity such as acceleration, angular acceleration, angular velocity, or load.
2. Description of the Related Art
A known acceleration sensor including piezoelectric vibrators is disclosed in Japanese Unexamined Patent Application Publication No. 2002-243757 filed by the assignee/applicant of this application.
The known acceleration sensor outputs an acceleration detection signal in the following manner. A bridge circuit is configured with two piezoelectric vibrators receiving stresses generated by acceleration in opposite directions and two load impedances each including a capacitor. A voltage-dividing impedance circuit is provided between the average output terminals of the bridge circuit. An oscillating circuit is configured by feeding back a signal from the voltage-dividing point of the voltage-dividing impedance circuit to the connection point of the two piezoelectric vibrators by a feedback signal processing circuit. The phase difference between oscillation output terminals from the average outputs of the bridge circuit is detected as an acceleration detection signal.
Since the acceleration sensor according to Japanese Unexamined Patent Application Publication No. 2002-243757 includes a bridge circuit configured with two piezoelectric vibrators and two load impedances each including a capacitor, the phase difference between oscillation outputs does not equal zero unless the bridge is at equilibrium. In other words, even if the stresses applied to the two piezoelectric vibrators both equal zero, the output of the acceleration sensor does not equal zero. Furthermore, the known acceleration sensor has a problem in that it is difficult to control the phase shifting circuit so that acceleration sensitivity is maximized.
Since the piezoelectric vibrators and the circuits are expected to be disposed more than 10 cm apart, a detection method that does not cause problems even when the piezoelectric vibrators and the circuits are disposed more than 10 cm apart has been urgently needed.
The assignee/applicant of this application has disclosed in Japanese Unexamined Patent Application Publication No. 2003-254991 a mechanical quantity sensor that has solved the above-identified problem. The mechanical quantity sensor includes two piezoelectric vibrators receiving stresses generated by acceleration in opposite directions, a voltage signal applying circuit configured to apply a common voltage signal to the two piezoelectric vibrators, an electric current-to-voltage converter circuit configured to convert the current signals flowing through the piezoelectric vibrators into voltage signals, and a phase difference signal processor circuit configured to detect the phase difference between output signals from the electric current-voltage converter circuit and to output a mechanical quantity signal.
The mechanical quantity sensor according to Japanese Unexamined Patent Application Publication No. 2003-254991 is described with reference to FIG. 9.
In FIG. 9, an acceleration detection element 10 includes two piezoelectric vibrators Sa and Sb receiving stresses generated by acceleration applied in opposite directions. The piezoelectric vibrators Sa and Sb are connected in series to resistors RLa and RLb, respectively. A current-to-voltage converter/signal adder circuit 11 converts the current signals flowing through the piezoelectric vibrators Sa and Sb of the acceleration detection element 10 into voltage signals so as to output an Sa signal and an Sb signal, respectively. Furthermore, the current-to-voltage converter/signal adder circuit 11 outputs an added signal obtained by adding the Sa and Sb signals.
A voltage amplifier/amplitude limiter circuit 12 amplifies the voltage of the added signal, limits the amplitude, and outputs a voltage signal Vosc to the acceleration detection element 10. The voltage signal Vosc is applied to a common connecting point of the piezoelectric vibrators Sa and Sb.
A phase-difference-to-voltage converter circuit 13 generates a voltage signal that is proportional to the phase difference between the Sa and Sb signals converted into voltage signals.
An amplifier/filter circuit 14 amplifies the voltage signal converted by the phase-difference-to-voltage converter circuit 13 by a predetermined gain, eliminates unwanted frequency band components, and outputs the obtained signal as an acceleration detection signal.
In the circuit shown in FIG. 9, the resonant frequencies of the piezoelectric vibrators Sa and Sb are made equal so that the frequency Vosc is a resonant frequency fr(0) for both of the piezoelectric vibrators Sa and Sb. In this way, when stresses having reversed phases, such as compression (pulling) and pulling (compression), are applied to the piezoelectric vibrators Sa and Sb, respectively, an output signal can be obtained from the amplifier/filter circuit 14.
The Vosc is a feedback voltage signal of a self-excited oscillator circuit configured of a loop of the piezoelectric vibrators Sa and Sb, the current-to-voltage converter/signal adder circuit 11, and the voltage amplifier/amplitude limiter circuit 12.
As illustrated in FIG. 9, resistors RLa and RLb are connected in series to piezoelectric vibrators Sa and Sb, respectively. Therefore, the damping ratio increases, and, thus, the change rate of acceleration detection sensitivity can be reduced in a wide temperature range. As a result, the sensor can be stabilized with respect to environmental temperature.
FIG. 10A shows the relationship between the magnitude of a resistor connected to the piezoelectric vibrators and the change rate of the temperature characteristics of the acceleration detection sensitivity (G sensitivity). The value of the damping ratio represented by the horizontal axis is obtained as ‘damping ratio=RL/resistance at resonance,’ where RLa=RLb=RL, when the resonance of the piezoelectric vibrators at resonant frequency is defined as the resonant resistance. The vertical axis represents the change rate range ((maximum value)−(minimum value)) of the acceleration detection sensitivity for the entire operational temperature range (−40° C. to +85° C.). As the damping ratio is increased, the change rate range of the acceleration detection sensitivity decreases and stabilizes with respect to temperature change.
According to an experiment carried out by the inventor, the sensor operated normally when the damping ratio was 2. However, when the damping ratio was increased to 6, abnormal oscillation was observed and the sensor was incapable of normally operating as a mechanical quantity sensor. The abnormal oscillation was caused by a decrease in response at a predetermined oscillation frequency due to an increase in the damping ratio, reducing the difference to the response at an unwanted oscillation frequency.
FIGS. 10B and 10C show the frequency characteristics of the open loop gain of a self-exciting oscillating circuit including the acceleration detection element 10, the current-to-voltage converter/signal adder circuit 11, and the voltage amplifier/amplitude limiter circuit 12, shown in FIG. 9, where the damping ratio is 2 and 6, respectively. Here, ‘S’ represents the response at a predetermined oscillation frequency and ‘N’ represents the response at an unwanted oscillating frequency band higher than the frequency N with a high gain. In general, to prevent abnormal oscillation, the difference between the gain Gs of the response S at the predetermined oscillation frequency and the maximum gain Gn of the response N at the unwanted frequency band must be 10 dB or more. In this known example, when the damping ratio was 2, the difference was 11.5 dB, which did not cause abnormal oscillation. However, when the damping ratio was 6, the difference is 6.3 dB, which caused abnormal oscillation.
The above-described problem is not limited to a sensor for detecting acceleration but is a problem that is common to sensors in which electric currents flow through piezoelectric vibrators in accordance with a mechanical quantity, such as angular acceleration, angular velocity, or load.
To minimize the signal intensity in the unwanted frequency band, a frequency filter may be provided in the oscillation loop of the self-exciting oscillating circuit. However, since a frequency filter has phase characteristics, the rate change of the phase (slope of the phase) with respect to a frequency change of a feedback signal is steep. Moreover, since there is a fluctuation in the phase characteristics of the frequency filter, if only the frequency filter is provided, the effect of the fluctuation of the phase characteristics of the frequency filter will be great, causing a new problem in that fluctuation and temperature change rate of the detection sensitivity will be increased.